20 Février – Thesis defense - Gabriel Amiard-Hudebine

14 h Amphi 2 - building A9 (University of Bordeaux / Talence campus)

Development and applications of nanosecond and picosecond lasers sources.

This thesis is separated in two separate studies. The first part reports the study, the realization and the applications of a compact nanosecond laser amplifier chain for the ignition of turboshaft engines. Anxious to offer more efficient engines that respect the environmental constraints imposed by the legislation, engine manufacturers seek to extend the ignition range of their engines to low pressures and low temperatures while guaranteeing ignition with an air / kerosene mixture ratio the lowest possible. This would enable an engine to relight in flight. However, the current ignition technology based on arcing candles is reaching its limits, and among non-conventional surrogate techniques, non-resonant breakdown laser ignition based on compact infrared sources is particularly promising. This part of the thesis first describes the study of a laser amplifier chain delivering at a rate of 100 Hz, nanosecond pulses with an energy of 10 mJ and improvements to increase this energy up to 20 mJ. The results obtained on each of the amplifier stages are validated by a numerical model which makes it possible to take into account the transient gain generation scheme in a quasi continuous pumped amplified medium. This thesis part also proposes a numerical modelling of the formation of a dynamic thermal lens resulting from a quasi continuous pumping inside an amplified medium. Finally, this part of the thesis reports on two test campaigns on the MERCATO bench at ONERA. During these tests, this source was able to ignite for the first time an aeronautical injector under conditions of pressure and temperature representative of ignition at high altitude. Moreover, this nanosecond source validated an ignition at ambient temperature and ambient  pressure with the same mixture ratio as in with a conventional arc candle. As a result, the proof was given that non-resonant breakdown ignition is a serious and promising alternative to the spark plug.
The second part of this manuscript deals with the study of an optical parametric oscillator (OPO) using a periodically polarized lithium niobate crystal (PPLN) pumped by a picosecond laser source. The pump is an ytterbium doped fibered oscillator with passive mode-locking in normal dispersion mode capable of delivering at a rate of 108 MHz ,pulses centred at 1035 nm with an average power of 4 W and a pulse duration of 1 ps. This duration can be reduced to 380 fs by adding a compression module of two chirped mirrors at the output of this laser. The OPO described in this manuscript is intended to add up to the spectroscopic arsenal of the molecular spectroscopy platform COLA of the Laboratoire d'onde et matière d'Aquitaine. This OPO converts pump pulses of 1 ps centred at 1035 nm over a spectral range from 1.4 μm to 1.8 μm with a pump-signal conversion rate of 30%. The pulses generated by this OPO are systematically characterized spectrally and temporally. Finally, this study deals with an original phenomenon allowing the OPO to generate short signal pulses and (quasi) limited by Fourier transform from pump pulses with temporal chirp. This self-compression is obtained by adjusting the synchronism of the pump and signal pulses. The OPO is thus able to generate signal pulses having a duration of 220 fs or 75 fs from pump pulses of 1 ps or 380 fs. Although this compression is done at the expense of the average power of 20 to 40%, the gain in peak power is of the order of a factor of three.

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